The following are the principal parts governing the nuclear power plant operators.
These cells have three principal parts: the dendrites, the cell body, and one axon.
The principal parts of the device are a light unit, positioning arm and firmware timer to control duration of exposure.
Patents and patent applications consist of two principal parts.
British Dictionary definitions for principal parts
(grammar) the main inflected forms of a verb, from which all other inflections may be deduced. In English they are generally considered to consist of the third person present singular, present participle, past tense, and past participle